Elements pool and drag drop (#2196)

* Implemented drag-and-drop composition

* Added composition policy for tables

* Reimplemented elements pool in Vue

* No need to resolve all objects on the navigated path

* Only show elements pool in edit mode

* Remove old elements pool

* Updated legacy code to use composition policy API

* Keep object in sync when mutated

src/api/Editor.js

@@ -36,6 +36,9 @@ export default class Editor extends EventEmitter {
      * or finish() are called.
      */
     edit() {
+        if (this.editing === true) {
+            throw "Already editing";
+        }
         this.editing = true;
         this.getTransactionService().startTransaction();
         this.emit('isEditing', true);